Salmonella-induced cholesterol accumulation in infected macrophages suppresses autophagy via mTORC1 activation

Salmonella enterica serovar Typhimurium is a Gram-negative bacillus that infects the host intestinal epithelium and resident macrophages. Many intracellular pathogens induce an autophagic response in host cells but have evolved mechanisms to subvert that response. Autophagy is closely linked to cellular cholesterol levels; mTORC1 senses increased cholesterol in lysosomal membranes, leading to its hyperactivity and suppression of autophagy. Previous studies indicate that Salmonella infection induces dramatic accumulation of cholesterol in macrophages, a fraction of which localizes to Salmonella containing vacuoles (SCVs). We previously reported that the bacterial effector protein SseJ triggers cholesterol accumulation through a signaling cascade involving FAK and Akt, however the importance of cholesterol accumulation to bacterial survival has remained unknown. Here we show that mTORC1 is recruited to SCVs and is hyperactivated in a cholesterol-dependent manner. If cholesterol accumulation is prevented pharmacologically or through mutation of sseJ, autophagy is induced and bacterial survival is attenuated. Notably, the host lipid transfer protein OSBP is also recruited to SCVs and its activity is necessary for both cholesterol transfer to SCVs and mTORC1 activation during infection. We propose that S. Typhimurium induces cholesterol accumulation through SseJ to activate mTORC1, preventing autophagic clearance of bacteria. We also conducted the first known lipid mass spectrometry analysis of S. Typhimurium infected macrophages. We found that S. Typhimurium manipulates macrophage lipid homeostasis via the SPI-2 effector SseJ, which is necessary for the increase in lysophosphatidylcholine and cholesterol ester species found in infected macrophages. This lipid remodeling likely facilitates bacterial persistence by promoting an anti-inflammatory M2 polarization state, which is advantageous for long-term Salmonella survival. Our findings provide novel insights into the interplay between S. Typhimurium and macrophage lipid homeostasis, highlighting lipidomic reprogramming as a mechanism for bacterial survival and immune modulation.